Vehicle windows are commonly printed around their peripheral margins with so-called obscuration bands. These are opaque, usually black, and may cover the rough vehicle body parts, wires etc which underlie the peripheral margin of the window, or may help to protect the adhesive bonding the window to the vehicle body from UV degradation.
The printing on to a vehicle window is normally done using a silk screen process. The screens are prepared to be selectively permeable to ink. Some areas of the screen are blocked out and other areas are left open. The open, ink permeable areas correspond to the patterns, for example, the obscuration band, which are to be printed on the glass.
The preparation of a printing screen involves stretching woven fabric, for instance, polyester, tightly across a frame, often of aluminium. The blocking may be done using a photographic technique. In one such technique, the screen is coated with photosensitive emulsion either by hand or by machine. After this, artwork is fixed, say by vacuum holding means, against the screen. The artwork may be in the form of a transparency, prepared and printed using, for example, CAD. The transparency has masked out, opaque areas which match the desired printed pattern Consequently, when the screen is subsequently exposed to light, the masked out areas prevent any light getting through to the emulsion underneath. This unexposed emulsion remains soft and can be washed away with warm water jets leaving the fabric therebelow open and permeable to ink. The exposed emulsion hardens so as to render the fabric it coats impermeable. The screen therefore ends up impermeable everywhere except in the areas which correspond to the desired printed pattern.
The printing of a window obscuration band is commonly carried out as part of the window production process, prior to bending and toughening or laminating. In the printing machine, the screen is suspended horizontally above the glass. The machine has a flood coater and a squeegee, each of which makes a pass across the screen. On a first pass, the flood coater coats the screen with ink. On the next pass, the squeegee forces the screen to make a line contact with the glass. Where contact is made by an open screen area, the ink carried therein is transferred on to the glass below. Initially, the ink is transferred in the form of discrete pillars, each pillar corresponding to the blob of ink carried in a particular pore in the screen mesh. In time, the discrete pillars spread and fuse into their adjacent neighbours to form a continuous coating of ink. The coating is then cured or dried.
There is an increasing tendency for vehicle designers to specify more exposed edges to their vehicle windows, and this in turn places demands on the window manufacture to be able to print close to the window edge. There are, however, a number of difficulties associated with printing to the edge. For example, mis-registration of the printing screen and the glass, or variations in the size of the glass, can result in ink being transferred from the screen on to the edge of the glass or the ink may spread over the edge. This is undesirable, both for aesthetic reasons and because the excess ink can result in the glass being weakened on toughening. One way of avoiding this has been to aim to print the band slightly short of the edge. However, this is not an altogether satisfactory method because it can in certain circumstances result in there being a noticeable gap between the band edge and the glass edge.
Other methods of printing to the edge have been proposed but they generally tend to involve complex, specialist apparatus and increased processing times. For instance, in EP 507 643 it is proposed to place an extension piece around the glass, to print the glass beyond its edge and on to the extension piece and then to remove the extension piece once the ink has dried. What is required is a solution to the problem of printing to the edge which does not involve complex or lengthy processing.